MRI is an effective, non-invasive magnetic imaging technique for generating sharp images of the internal anatomy of the human body, which provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumors and the like. Briefly, the patient is placed within the center of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed radio-frequency (RF) magnetic field is then applied causing the protons to begin to precess around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signals, which are detected by sensors of the MRI system. Based on the RF signals emitted by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
A significant problem with MRI is that its strong magnetic fields can interfere with the operation of any medical devices, particularly pacemakers or ICDs, implanted within the patient. Typically, pacemakers and ICDs include pulse generators for generating electrical pacing pulses and shocking circuits for generating stronger defibrillation shocks. A set of conductive leads connect the pulse generators and shocking circuits to electrodes implanted within the heart. An individual pacing pulse is applied by using the pulse generators to generate a voltage difference between a pair of the electrodes, such as between a tip electrode implanted within the right ventricle and the pacemaker housing or “can.” A defibrillation shock is applied by using the shocking circuits to generate a much larger voltage difference between a pair of the electrodes, such as between a large coil electrode implanted within the right ventricle and the pacemaker housing. The leads may also have a variety of sensors for sensing physiological signals within the heart of the patient, such as pressure sensor, temperature sensors, SvO2 sensors, PPG and the like. The sensors are typically connected to the implantable device via electrical signal conduction paths within the various leads so as to receive control signals from the implanted device and to relay sensed signals back to the device. The pulse generators, shocking circuits, leads, electrodes and sensors, as well as the tissue and fluids between the electrodes and sensors, collectively provide various conduction loops. State of the art pacemakers and ICDs exploit lead systems having numerous electrodes and sensors, thus presenting numerous possible conduction paths.
When patients with pacers or ICDs are exposed to MRI fields, RF fields of the MRI can induce currents along the conduction paths causing excessive current to flow into tissue or blood through tip or ring electrodes resulting in Joule heating. Excessive power dissipation might damage the tissues around pacing electrodes causing inappropriate sensing and pacing and posing risks to the patient. Additionally, the powerful gradient fields of an MRI system can induce currents among the conduction paths sufficient to trigger rapid, unwanted pacing pulses or even defibrillation shocks. These induced currents are referred to as parasitic currents. Rapid pacing pulses induced by the MRI could, in certain cases, cause a life-threatening fibrillation of the heart. Likewise, any defibrillation shocks triggered by the presence of the MRI fields can also induce fibrillation, particularly if the shock is delivered during a repolarization period of the ventricular myocardium. Another significant concern is that the induced voltages can be mistakenly sensed by the pacemaker as intrinsic heartbeats. In some pacing modes, particularly demand-based modes, the pacemaker then assumes that the heart needs no pacing assistance and will block its pacing output (i.e. delivery of a pacing pulse is inhibited.) This could cause a “pacing dependent” patient to pass out and possibly die.
In view of these concerns, various safeguard techniques have been developed that operate to detect the strong fields associated with an MRI and then switch sensing modes or pacing modes in response thereto. See, for example, U.S. Patent Application 2003/0083570 to Cho et al.; U.S. Patent Application 2003/0144704 to Terry et al.; U.S. Patent Application 2003/0144705 to Funke; U.S. Patent Application 2003/0144706 also to Funke; U.S. Pat. No. 6,795,730 to Connelly et al., and U.S. Patent Application 2004/0088012 of Kroll et al.
However, it would be preferable to allow the implanted device to continue to operate in its normal pacing modes even during an MRI procedure, so long as heating criteria is met, arrhythmias are not induced, unnecessary pacing pulses or shocks are not delivered, and any necessary therapy is not improperly inhibited. That is, it would be desirable to allow the device to continue to monitor the heart of the patient for arrhythmias or other medical conditions even during an MRI procedure and to deliver therapy as needed and to transmit signals to deactivate the MRI system only if absolutely necessary. With such a system, it would also be desirable to control the device to transmit monitoring and diagnostic information during the MRI procedure to an external monitoring and control system so that medical personnel can monitor the status the implanted device and the health of the patient during the MRI procedure. The medical personnel then could deactivate the MRI system if warranted or adjust its operation if needed. The medical personnel could also re-program the operation of the implanted device during the MRI procedure, if appropriate. The implanted device would preferably also monitor for any arrhythmias or other abnormal medical conditions induced by the MRI fields and send appropriate signals to the MRI system to automatically deactivate the MRI system. In particular, the device would monitor for any tachyarrhythmias induced by the MRI fields so that the MRI system can be promptly deactivated and appropriate therapy delivered.
With conventional implantable systems, though, the strong MRI fields can prevent the implanted device from reliably sensing signals from the various electrodes of the leads and from the various physiological sensors, thus preventing the implanted device from reliably detecting arrhythmias or other abnormal conditions within the patient during the MRI procedure. Accordingly, there is a need to provide improved implantable components configured to allow the implanted device to continue to reliably receive signals from sensing leads and physiological sensors during an MRI and it is to this end that certain aspects of the invention are directed. Moreover, the strong MRI fields can also prevent the implanted device from reliably sending transmissions to, and receiving signals from, an external system, thus preventing the implanted device from reliably sending diagnostics data and warning signals to the external system during the MRI procedure and possibly also preventing the external system from transmitting re-programming commands to the implanted device during the MRI procedure. Accordingly, there is also a need to provide improved implantable components and external components sufficient to allow the implanted device and the external system to reliably communicate with one another during an MRI procedure and it is to this end that other aspects of the invention are directed.
Still another significant concern is that the MRI fields can cause tip electrodes of the leads to become significantly heated, potentially damaging adjacent tissues. Techniques have been developed for detecting the heating of tip electrodes and deactivating the MRI system in response thereto. See, for example, U.S. Patent Application 2006/0025820, of Phillips et al., entitled “Integrated System and Method for MRI-safe Implantable Devices.” Typically, though, the implanted device merely determines whether the tip temperature has exceeded a threshold and sends signals to deactivate the MRI system. It would be preferable to additionally track changes in tip temperatures so as to provide other diagnostic information and, in particular, to exploit changes in tip temperature to determine the amount of current induced in a given lead by the MRI fields. It is to this end that still other aspects of the invention are directed.